The present invention is directed thermal ink jet inks, and, more particularly, to the reduction of kogation on the resistor surface by the addition of copper-containing ions to the ink.
Small drop volumes, on the order of 10 picoliters (p1) or less, are advantageous for superior image and text quality. Smaller drops of ink on the paper (dots) are less visible and give printed images less of a grainy appearance. Because the colorant is in smaller xe2x80x9cunitsxe2x80x9d, then gradients in color tones are more continuous. Because individual dots are less visible, color text is also improved with smaller drops.
Long-life thermal inkjet print heads with replaceable ink supplies provide value to the customer because the need to replace expensive to manufacture print heads is minimized or eliminated. Replaceable ink supplies can be in the form of, but not limited to, off-axis ink reservoirs or ink tanks that snap onto the top of the print heads.
Formulation of a color ink set for a printer with small drop volume, superior color image and text quality, long-life print heads, and high throughput is a challenge. Smaller drop volume means more drops need to be fired for a given volume of ink and the temperature of the resistor surfaces of the small drop volume pens tend to be higher. Therefore, kogation is more of a problem. In limited life print heads, which are replaced with the ink supply (xe2x80x9con-axisxe2x80x9d), the print head can be replaced before kogation builds up to the point where it affects color image and text quality. In long-life print heads with replaceable ink supplied (xe2x80x9coff-axisxe2x80x9d), resistors must withstand hundreds-of-millions up to tens-of-billions of firings without kogation buildup.
Kogation is thought to be a complex process in which organic material, typically dyes, in the ink forms a graphitic film that builds up on the resistor surface during repeated ejection of drops. The koga can also be inorganic in nature but these types of koga are outside of the range of discussion here. Along with the buildup process, removal processes are also thought to occur that help to clean the resistor surface. These removal processes can include mechanical breakup and chemical oxidation of the koga. Mechanical breakup during the stresses of drop generation and vapor bubble collapse is not necessarily desirable because a rough koga surface can result that act as a xe2x80x9cboiling chipxe2x80x9d. The rough surface provides may low-energy nucleation sites for drive bubble formation early in the resistor pulse cycle. These premature vapor bubbles are associated with weak and erratic drop velocities and weights. More consistent and strong drop velocities and weights are associated with delayed drive bubble formation from smooth resistor surfaces.
Because of conductive heat dissipation away from the resistor, there is a sharp temperature gradient or profile on the resistor surface with the highest temperature in the center of the resistor and lower temperatures towards the perimeter. The temperature gradient is revealed by the heavier koga or tantalum oxidation in the center xe2x80x9chot spotsxe2x80x9d of the resistors after several million firings.
Metal ions can have a strong influence on kogation. Metal ions can be introduced into thermal inkjet inks through: (1) impurities and contamination in ink manufacture; (2) corrosion of metal parts in ink filling equipment; (3) corrosion of metal parts within the thermal inkjet pen such as stainless-steel springs and screens; (4) deliberate addition of metal salts or metal-organic complexes for, e.g., controlling bleed; and (5) metallized dyes, such as copper phthalocyanines.
Kogation is also strongly affected by the metal type.
There remains a need for a process of eliminating, or at least reducing, kogation on the firing resistor of an inkjet print head.
In accordance with the present invention, a thermal inkjet ink for inkjet printing is provided, the ink comprising at least one colorant and a vehicle. The inkjet ink is provided with a copper ion concentration within a range of about 30 to 1,000 ppm, based on the total ink composition.
The presence of the copper ion reduces kogation, and is especially effective in inkjet pens that either jet low drop volumes (xe2x89xa610 picoliters) or have a prolonged resistor life, such as off-axis print cartridges.